Compact, low cost raman monitor for single substances

ABSTRACT

Apparatus for performing Raman analysis may include a laser source module, a beam delivery and signal collection module, a spectrum analysis module, and a digital signal processing module. The laser source module delivers a laser beam to the beam delivery and signal collection module. The beam delivery and signal collection module delivers the laser source beam to a sample, collects Raman scattered light scattered from the sample, and delivers the collected Raman scattered light to the spectrum analysis module. The spectrum analysis module demultiplexes the Raman scattered light into discrete Raman bands of interest, detects the presence of signal energy in each of the Raman bands, and produces a digital signal that is representative of the signal energy present in each of the Raman bands. The digital signal processing module is adapted to perform a Raman analysis of the sample.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/923,571, filed Oct. 24, 2007; which claims benefit under 35 U.S.C.§119(e) of provisional U.S. patent application No. 60/854,339, filedOct. 24, 2006, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND

Raman analysis is a well-known analytical technique used for structuralanalysis of molecular species. Raman analysis exploits the inelasticscattering of light by a substance that carries information about thevibrational spectrum of the constituent molecular compounds. Thedistinguishing feature of Raman analysis compared with, for example,infrared (IR) or near infrared (NIR) absorption vibrational spectroscopyis the fact that the optical excitation of the sample is done at a muchshorter wavelength of light and the corresponding Raman signal isemitted in the spectral band of a much shorter wavelength than the IRabsorption lines of the same sample. This feature allows efficientanalysis of aqueous solutions and also makes it possible to constructvery compact instruments.

Typical Raman systems employed so far use a monochromatic source ofincident radiation such as a laser combined with a spectrometer and adetection system such as a CCD camera (dispersive Raman). Another typeof a conventionally used Raman system is Fourier transform Raman thatuses a scanning interferometer and a single detector for spectralanalysis. Lasers that are used for Raman analysis typically emit in thevisible, near infrared, or near ultraviolet spectrum. Many variations ofradiation detectors have been used in the art, ranging fromphotomultiplier tubes to CCD cameras.

Traditionally, the cost of Raman spectrometers has been driven by thecost of the laser excitation source, the spectrometer that may use notchfilters designed to suppress Rayleigh scattering, cooled CCD camera andthe signal analysis module that usually requires a fully functionalcomputer, operating system and specialized software. Some Raman systemsalso employ a fiber-optic probe for delivering laser light to the sampleand collecting the Raman signal.

SUMMARY OF THE INVENTION

The invention provides apparatus and methods for Raman analysis usingthree-dimensional Bragg grating elements in combination withcommercially available lasers and detectors to perform quantitativeanalysis of various types of samples with respect to select substancesof interest. Three-dimensional Bragg gratings have been previouslydescribed, for example, in U.S. Pat. No. 7,031,573, the disclosure ofwhich is incorporated herein by reference.

By utilizing three-dimensional Bragg grating elements the size and costof a Raman system can be drastically reduced. Three-dimensional Bragggrating elements can be used to produce a stable laser source suitablefor Raman analysis out of mass-produced laser diodes that are used, forexample, in CD and DVD drives. They can also be used to filter Rayleighscattering by manufacturing ultra-narrow notch filters using thattechnology, and to analyze the Raman spectra scattered from the samplematerial. Both reflective and transmissive Bragg gratings can be used inconstruction of various embodiments of the disclosed invention.

By combining three-dimensional Bragg grating elements with mass-producedlasers and detectors, high-quality Raman-scattering sensors can beproduced at a reasonable cost, making more accessible their use ineveryday applications such as airport security, law enforcement, medicaldiagnostics, manufacturing quality testing, explosive detection etc. Thethree-dimensional Bragg grating elements enable the use of much smallercomponents then used in conventional dispersive Raman systems, which canresult in Raman instruments as small as a few cubic centimeters in size.Smaller instruments can be used in Raman sensor devices such as wearablepersonal detection devices.

Such Raman monitors can be tuned to detect specific molecular species ofinterest and perform quantitative analysis of their content. Bymonitoring only certain bands of the Raman spectral range where peaksfor select substance(s) of interest appear, small Raman instrumentsenabled by the technology of the three-dimensional Bragg gratings can bebuilt to detect the presence of the target substance(s). For example, itmay be desirable to build sensors capable of alerting the user of thepresence of a harmful contaminant or bacteria such as Anthrax. Thepotential usefulness of such targeted sensor devices in applicationsrelated to monitoring for toxins, spores, pollutants and allergens isevident.

By exploiting known Raman spectra of different substances, devices canbe built using three-dimensional Bragg gratings that can analyze thecontent of select substance(s) in a mixed sample that producesconvoluted Raman spectra. For example, in a very simple case when asample contains two substances, one with a characteristic spectrum offive peaks in five spectral bands and one with six peaks in otherspectral bands, an instrument can be developed using three-dimensionalBragg gratings as filters. If the filters are placed such that all fivepeaks appear in the five detectors, the device unambiguously identifiesthe first substance. In more complex cases a statistical analysis may becarried out using appropriate methods (including but not limited topartial least squares, multiple linear regression, neural nets,principal component regression etc.) to select best parameters forquantitative analysis.

In a simple example, the positions of the valleys in the Raman spectrumof the molecular species of interest also carry important information.For that reason one band or multiples bands may be added to the spectrumanalysis module that correspond to the position of the spectral valleys.These bands are important to differentiate the Raman signal from itsbackground that may be caused, for example, by fluorescence. Thesespectral bands allow providing quantitative information about the amountof the detected substance in the sample.

In some embodiments, inelastically-scattered light is separated fromelastically-scattered light by a signal separation block to yield theRaman signal. The Raman signal is separated (or demultiplexed) intospecific bands by a spectrum analysis module. The spectrum analysismodule, which in some embodiments is a demultiplexer, may be constructedusing three-dimensional Bragg grating elements to direct each of theselected spectral bands of the Raman signal to a detector that monitorsthe intensity of that selected spectral band. In different embodiments,demultiplexers using three-dimensional Bragg gratings or other suitabletechnologies can be used to separate the various spectral bands of theRaman signal and direct each band to the appropriate detector formonitoring. A multi-band filter or several such filters can also beconstructed using the technology of three-dimensional Bragg gratings tofilter out multiple select bands from the broad band signal and directthe filtered light to a detector or several detectors. Optimalconstruction of such filters will depend on the results of statisticalanalysis of a particular chemometrics problem with respect to the bestcombination of parameters for quantitative prediction.

A variety of software functions that can also be implemented inprogrammable logic circuits may be used to process the signal from theRaman sensor instrument and provide the detection of species in thesample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B provide examples of Raman spectra for varioussubstances.

FIG. 2 is a functional block diagram of a Raman spectrometer thatincludes a plurality of three-dimensional Bragg grating elements.

FIG. 3 is a diagrammatic view of a Raman tester that includes aplurality of three-dimensional Bragg grating elements.

FIG. 4 depicts an example sensor packaged for commercial application.

FIG. 5 provides Raman spectra produced by two laser sources withdifferent emission wavelengths.

FIG. 6 depicts an example circuit for analysis of a collected signal.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Detection methods based on Raman analysis have superior specificitysince any molecule unique in structure will have a corresponding uniquecombination of vibrational modes. However, among the multitude of thesemodes there is usually a significantly smaller number that bestdistinguish the unique character of the molecule. These modes maycorrespond to certain collective motions of the atoms unique to theirspatial positions relative to each other. These modes may, therefore, becalled “Raman signature lines” of a particular molecule. The number ofthese signature lines may be different for each molecule but it isgenerally a substantially smaller number than that contained in itsentire Raman spectrum. For example, FIG. 1B shows Raman spectra of lightalcohols that clearly show that a few select spectral bands can be usedfor differentiation between these substances. Successful detection ofpractically any molecule can be carried out by performing correlation ofonly a small subset of the Raman-active lines in the vibrationalspectrum of a sample under study with that of a pure substance ofinterest.

This approach greatly simplifies the design of a Raman sensor and allowsefficient use of inexpensive components, such as mass-produced laserdiodes, PIN photo-diodes, and three-dimensional Bragg gratings. It alsoeliminates the use of a computer with operating system and sophisticatedsoftware that typically add cost, consume a lot of power and typicallyrequire long times for processing of each sample.

The components of a Raman sensor can be realized by combiningmass-produced semiconductor lasers and detectors with three-dimensionalBragg grating elements. For example, a Raman laser source and Ramanspectrum detection system can be produced using such combinations.Because these components are small in size, Raman instruments can beproduced that are exceptionally small, even as small as a few cubiccentimeters. Due to wafer-scale efficient reproduction methods forthree-dimensional Bragg grating elements, the cost of these instrumentsmay be kept very low that enables their application as Raman monitors ina wide range of areas including buildings, public spaces, vehicles, andeven personal devices.

By using this technology, Raman sensors may be designed that monitorspecific spectral lines that may, for example, corresponding to Ramansignature of any substance(s) of interest and a particular laserexcitation wavelength. Another example of Raman spectra of severalcompounds is shown in FIG. 1A. Different spectral patterns are shown forthe picolinic acid produced by bacterial spores such as anthrax(bacillus cereus), chalk, aspirin, and creamer. The distinct pattern ofRaman signature lines for the substance associated with bacteria sporescan be readily distinguished. These five prominent Raman bands can thusbe used to differentiate bacterial spores from other white powders in anembodiment of the invention that is adapted to serve as a Raman monitorfor this substance. By using five Bragg grating elements, each of whichallows passage of one and only one of the spectral bands of the Ramansignature into each of five detectors, a signal from the five detectorscan unambiguously identify the substance by its characteristic Ramanlines. Thus, a device according to the invention may be adapted tomonitor for a specific substance or several substances.

Additionally, such a device may be adapted to detect spectral bands thatcorrespond to valleys (or areas of absence of Raman peaks) in the Ramanspectrum of substance(s) of interest. This allows monitoring theintensity of a background signal, fluorescence, or any othernoise-contributing signals. Determining background signal level may beuseful in quantitative monitoring of a substance of interest throughcalculations that subtract the noise contribution from the signal, forexample, to determine the concentration of the analyte in a sample.

FIG. 2 is a functional block diagram of an example embodiment of a Ramananalysis system 100 comprising a plurality of three-dimensional Bragggrating elements. Examples of such three-dimensional Bragg gratings, andmethods for making the same, are described, for example, in U.S. Pat.No. 7,031,573.

Such a system 100 may include a laser source 102 that emits laser light104. The laser source 102 may be a commonly found semiconductor 120-mW,785-nm laser source. It should be understood that laser source 102 mayemit laser light 104 of any desired wavelength, such as, for example,630 nm, 830 nm, 980 nm, or 1064 nm. The laser source 102 may be afrequency-doubled laser source, such as disclosed in U.S. patentapplication Ser. No. 10/884,524, the disclosure of which is incorporatedherein by reference. The laser source 102 may be an ultra-violet lasersource. The laser power may be in a range from mW to Watts, as desiredfor a particular application. The laser light 104 may be deliveredcontinuously or in pulsed mode. For more complex Raman analysis, severaldifferent wavelength sources may be combined. For example, by using twoclose-excitation wavelengths, it is possible to subtract thefluorescence background because it is independent of the excitationwavelength. This approach relies on the fact that Raman spectrum linesalways shift together with the wavelength of the excitation laser,whereas the fluorescence and other background sources are independent ofthe wavelength of the excitation laser.

Therefore, in this approach two laser sources with closely spacedemission wavelengths may be used. The separation between two wavelengthsmay be selected to approximately correspond to the width of the lines inthe Raman spectrum of the substance of interest. Raman scattering maythen be collected through the same optical system when the two lasersources excite the sample sequentially. In this case, as shown in FIG.5, the detectors aligned with different wavelength channels of thespectrum analysis module (demultiplexer) may detect the peaks of theRaman bands of the substance of interest when laser 1 is firing(designated “A” in FIG. 5) and detect the valleys of the same spectrumwhen laser 2 is firing (designated “B” in FIG. 5). Since the backgroundlight is independent of the excitation wavelength, its contribution tothe signal remains the same. As a result, when a subtraction of the twosets of measurements is performed the result is background-free set ofvalues representing true amplitudes of the Raman peaks of the selectedsubstance.

Based on this approach a simple circuit may be constructed for analysisof the collected signal. Such a circuit is depicted in FIG. 6. In thiscircuit 600, the collected data arrays are first subtracted, at 602, andthen directed to two channels—the summation channel 604 and thecorrelation channel 606. In the correlation channel 606, a simplecomputation of a correlation coefficient between the collectedbackground-free set of peaks and the stored array of true peakamplitudes for the pure compound of interest is performed. The output ofthe correlation channel 606 is a single value representing theprobability of identification of the selected compound in the sampleunder study. On the other hand, in the summation channel 604 the sum ofall true peak amplitudes is computed that is proportionate to the amountof the analyte present in the sample.

With reference once again to FIG. 2, a first three-dimensional Bragggrating element 106 may be positioned to receive the laser light 104.The element 106 may be adapted to condition the laser light 104, suchas, for example, by narrowing the bandwidth of the emitted laser light104. The element 106 may also be adapted to control the centralwavelength of the emitted light 104. By conditioning the emitted laserlight 104, the element 106 may adapt a commonly found semiconductorlaser 102 for use as a Raman source.

The conditioned laser light 108 may be received by a band pass filter110. The filter 110 may be adapted for “laser line” filtering. That is,the band pass filter 110 may allow the passage of a narrowband beam 112,which may have a bandwidth of, for example, approximately 1 nm. The bandpass filter 110 may suppress stray light outside the desired band. Theband pass filter 110 may include a thin film interference filter, forexample, or a three-dimensional Bragg grating element adapted to producea desired beam. Either a transmissive or reflective Bragg gratingelement may be used. The band pass filter 110 may have any desiredcharacteristics, such as width, transparency, side mode suppression,filter shape, physical size and shape, etc. The characteristics of thefilter may be adapted to determine stray light levels, sensitivity,resolution, and the like.

The narrowband beam 112 may be received by a beam delivery system 114.The beam delivery system 114 may include free-space delivery of acollimated beam to a sample S that is monitored at close range. The beamdelivery system 114 may include telescopic or microscopic optics. Thebeam delivery system 114 may include a fiber-optic beam delivery system,or any other optics system that is adapted to propagate a Raman signal.

The beam delivery system 114 may deliver Raman source light 116 to aRaman spectra sampling head 120. The Raman sampling head 120 may bedesigned to allow sampling at close range or from a distance. It isfurther possible to include Surface Enhance Raman Spectroscopy (SERS)substrates that can enhance the signal. The signal could also beenhanced by using methods of sample collection such as gaseous sampling,pumping, and swabbing of the sample. The sampling head 120 may beadapted for use with solid, liquid, or gaseous samples. The samplinghead 120 may come into direct contact with a sample S, or be coupled tothe sample S via a transparent window or other optical device. Thesampling head 120 may include telescopic or microscopic elements thatare suitable for remote sensing of the substance. Either the sample head120 or the sample S may be movable for such applications as rasterscanning.

A signal separator 118 may be adapted to separate the scattered lasersource light 116 from Raman signal light 122 that is scattered by thesample S. As shown, the signal separator 118 may be positioned betweenthe beam delivery system 114 and the Raman sample head 120. The signalseparator 118 may receive the Raman source light 116 from the beamdelivery system 114, and the Raman sampling head 120 may receive theRaman source light 116 from the signal separator 118.

The signal separator 118 may include a three-dimensional Bragg gratingelement, which may be adapted to perform as a narrowband notch filter.To reduce Rayleigh scattering that can enter the detection system, thenotch filter may allow passage only of light having a bandwidth thatmatches the line width of the excitation laser 102. Thus, stray lightmay be decreased, and the signal-to-noise ratio improved. Otherwell-known technologies may also be used, for example thing filmdielectric filters and dichromated gelatin holographic filters.

The signal separator 118 may be positioned to receive a Raman signal 122that is scattered from the sample S. The signal separator 118 may beadapted to separate inelastically-scattered light 124 in the Ramansignal 122 from elastically-scattered light at the same wavelength asthe laser source 102.

The signal separator 118 may be adapted to direct theinelastically-scattered light 124 into a demultiplexer 126. Thedemultiplexer 126 may include one or more Bragg gratings that cooperateto separate one or more Raman bands 128A-C from the received portion 124of the Raman signal 122. The demultiplexer 126 may include a pluralityof discrete Bragg grating elements. In such an embodiment, each Bragggrating element may have recorded thereon a respective Bragg gratingthat redirects any portion of the received light that falls within acertain desired Raman band. Each Bragg grating element may betransparent to the remainder of the light it receives. The demultiplexer126 may include a Bragg grating chip, which may be a single bulk ofmaterial having a plurality of Bragg gratings recorded therein. EachBragg grating may be adapted to redirect any portion of the receivedlight that falls within a certain desired Raman band, and to transmitthe remainder of the light it receives to another Bragg grating withinthe chip. Three-dimensional Bragg grating chips are disclosed in U.S.Pat. No. 7,125,632, the disclosure of which is incorporated herein byreference.

Each Raman band 128A-C may be directed through a respective Raman bandpass filter 130A-C. Each such filter 130A-C may be a band pass filterwith strong side mode suppression to further reduce stray light in thebeam 128A-C. The width of the passband and the form of the filter may bedesigned for a particular application. One or more Bragg gratingelements may be adapted for this purpose. The filters 130A-C may havedifferent characteristics, such as width, transparency, side modesuppression, filter shape, physical size and shape, etc. Thecharacteristics of the filter used will aid in determining thecharacteristics of the instrument. A number of Bragg grating filters maybe placed in series in order to further suppress stray light orinfluence the selectivity and resolution of the Raman monitor.

The filtered Raman bands 132A-C may be directed into a detector system134, which may include one or more detectors 136A-C. Each band 132A-Cmay be directed into a respective detector 136A-C. Each such detector136A-C may be adapted to determine a signal intensity level associatedwith the received Raman band 132A-C. The detectors 136A-C may bediscrete Si or InGaAs pin detectors, APD detectors, or the like.Detector arrays may be constructed in two or three dimensions to allowfor spectral imaging of the sample.

FIG. 3 is a diagrammatic view of a Raman tester 200 that includes aplurality of three-dimensional Bragg grating elements. As shown, a lasersource module 210 may include a laser source 212 and a Bragg gratingelement 216. The Bragg grating element may be adapted to condition thelaser beam 214 emitted from the laser source 212, as described in detailabove, to produce a Raman source beam 218.

The laser source module 210 may deliver the Raman source beam 218 to abeam delivery and signal collection module 220. The module 220 mayinclude a Bragg grating element 222 having a Bragg grating recordedthereon that is adapted to direct the Raman source beam 218 to a Ramansampling head 224.

The Bragg grating element 222 may be further adapted to receive anddeliver Raman scattered light 226 that is scattered from the sample S toa spectrum analysis module 230. As shown, the Bragg grating element 222may be adapted to perform as a narrowband notch filter that allowspassage of only certain bands of the Raman spectral range, e.g., wherepeaks for a given substance of interest may appear. As shown, a singleBragg grating element 222 may be used both to direct the Raman sourcebeam 218 onto the sample S and to deliver Raman scattered light 226 thatis scattered from the sample S to a spectrum analysis module 230.Alternatively, as illustrated in FIG. 4, a first Bragg grating element(such as one associated with a beam delivery and signal collectionmodule 320) may be used to direct the Raman source beam onto the sample,and a second Bragg grating element 322 may be used to deliver the Ramanscattered light to the spectrum analysis module.

The spectrum analysis module 230 may include one or more Bragg gratingelements 232A-C that cooperate to demultiplex the Raman scattered lightinto a plurality of discrete Raman bands of interest 234A-C, and todeliver the Raman bands 234A-C into respective detectors 236A-C. Thedetectors 236A-C may be adapted to detect the presence of signal energyin the Raman bands 234A-C. The detectors 236A-C may produce a digitalsignal 238 that is representative of the energy in the several Ramanbands. The spectrum analysis module 230 may pass the digital signal 238to a digital signal processor 240. The signal 238 may be processeddigitally to ascertain information about the Raman pattern. The DSP 240may include signal processing software or a programmable logic circuitthat is adapted to determine the qualitative and/or quantitativepresence of the substances measured. That is, the DSP module may beadapted to determine whether a substance is present in the sample,and/or how much of the substance is presenting the sample.

One channel (say, 232A→234A→236A) of the spectrum analysis module 230may be adapted to receive and monitor a background signal. Suchmonitoring enables the Raman spectra to be analyzed for quantitativeinformation after the background noise level is subtracted from energylevels in the Raman bands (leaving a more accurate measure of the truescattered energy in each of the Raman bands). One or more Bragg gratingelements that cooperate to allow passage or two or more passbands may beused. A first passband may be tuned to a desired Raman band, while theother passbands are tuned to bands where there is no Raman signal.Consequently, the ratio of signals from the respective passbands canprovide a quantitative measurement of the Raman signal by subtraction ofthe background and fluorescence signals from the Raman signal.

FIG. 4 depicts an example sensor 300 packaged for commercialapplication. The example embodiment shown includes a laser source module310, which may include a laser source and a Bragg grating element, asdescribed above in connection with FIG. 3. The Bragg grating element maybe adapted to condition the laser beam emitted from the laser source toproduce a laser source beam 318. A lens may be provided to collimate thebeam.

The laser source module 310 may deliver the laser source beam 318 to abeam delivery and signal collection module 320. As described above, thebeam delivery and signal collection module 320 may include a Bragggrating element and a Raman sampling head. The Bragg grating element mayhave a Bragg grating recorded thereon that is adapted to direct theRaman source beam 318 to a Raman sampling head. The Bragg gratingelement may be further adapted to receive and deliver Raman scatteredlight 326 that is scattered from the sample (not shown in FIG. 4) to aspectrum analysis module 330. The light 326 may also pass via the secondBragg grating element 322 to the spectrum analysis module 330 and, forexample, from the second Bragg grating element 322 through a lens 328.

The spectrum analysis module 330 may include a plurality of Bragggrating elements VBG that cooperate to separate respective Raman bandsof interest. The VBGs may redirect the Raman bands to respectivedetectors 336. As shown, the Raman bands may be directed from the Bragggrating elements VBG via reflectors 335 to the detectors 336. Thedetectors 336 may be adapted to detect the presence of signal energy inthe Raman bands. As shown, one channel 333 of the spectrum analysismodule 330 may be adapted to receive and monitor a background signal asdescribed in detail above.

As shown, such a sensor 300 may be a portable, compact device that isdedicated to detect a particular Raman spectrum of interest (e.g., aRaman spectrum for a substance expected to have peak energy in each offive bands). Note that such a sensor 300 may be about the size of arazor blade. Thus, such a monitor may be small and portable enough to beworn by a person or placed in open spaces, buildings, vehicles, tubes,pipes, or underground. Such a monitor may be designed to be operatedremotely, and may be configured to be operated by a battery, from a gridor electrical network, or from some other power source such as solarcells.

It should be understood that a Raman monitor according to the inventionmay be adapted to monitor for more than one substance by having asufficient number of spectral band monitors to detect the characteristicRaman spectra of more than one substance. Such “combination detectors”may be useful as “smoke” detectors in buildings or in securityapplications to screen for particular unwanted or harmful substances.Such combination detectors may also be useful to control manufacturingprocesses by monitoring hazardous by-products. Various modes of alarm,including audible sounds, visual cues, and vibrations, for example, maybe employed to alert a user to the presence of a monitored substance.

Various embodiments of the invention have wide application in the fieldof monitoring for hazardous substances. Monitoring substances such asbiohazards, explosives, poisons, and toxic gases using a Raman monitoris of interest to military and Homeland security applications. Lawenforcement could use Raman monitors tuned to monitor illicit substancessuch as illegal drugs and contraband. Industrial quality could bemonitored, with Raman devices monitoring the reactant and productstreams of processes in the chemical, pharmaceutical, petrochemical,food, and materials industries. Industrial applications could alsoinclude monitoring discharge streams for hazardous effluents, whethergaseous, liquid or solid. Water purification plants could also benefitfrom Raman monitoring.

Raman spectral monitors can also be used to identify materials such asplastics, metals, minerals, and any other material that provides acharacteristic Raman spectra. Possible medical applications using theseRaman monitoring devices include diagnostics that involve the Ramanspectra of various tissues or possible sampling to determine drug oftoxin metabolism or detection of counterfeit drugs. In general, theseRaman monitoring devices can be used to test almost any sample for thepresence or absence of one of more substances with a Raman signature.

What is claimed is:
 1. Apparatus for performing Raman spectroscopy on asample of interest, the apparatus comprising: a laser source adapted toexcite the sample sequentially at a first emission wavelength and asecond emission wavelength, wherein a separation between the first andsecond wavelengths corresponds to a width of a Raman band of a substanceof interest.
 2. The apparatus according to claim 1, further comprising acollector for collecting Raman scattering when the laser source excitesthe sample sequentially.
 3. The apparatus according to claim 1, furthercomprising a spectrum analysis module for performing Raman analysis ofthe collected Raman scattering based on a Raman spectrum associated withthe substance of interest.
 4. The apparatus of claim 3, wherein thespectrum analysis module includes a demultiplexer for demultiplexing theRaman scattering into discrete Raman bands of the substance of interest.5. The apparatus of claim 4, further comprising one or more detectorsaligned with different wavelength channels of the demultiplexer fordetecting peaks of the Raman bands of the substance of interest when thelaser source is exciting the sample at the first wavelength, and fordetecting valleys of the Raman bands of the substance of interest whenthe laser source is exciting the sample at the second wavelength.
 6. Theapparatus of claim 3, wherein a background-free set of valuesrepresenting true amplitudes of the Raman peaks of the substance ofinterest is produced.
 7. A method for performing Raman spectroscopy on asample of interest, the method comprising: exciting the samplesequentially at a first emission wavelength and a second emissionwavelength, wherein a separation between the first and secondwavelengths corresponds to a width of a Raman band of a substance ofinterest.
 8. The method according to claim 7, further comprisingcollecting Raman scattering when the laser source excites the samplesequentially.
 9. The method according to claim 7, further performingRaman analysis of the collected Raman scattering based on a Ramanspectrum associated with the substance of interest.
 10. The method ofclaim 9, further comprising demultiplexing the Raman scattering intodiscrete Raman bands of the substance of interest.
 11. The method ofclaim 10, further comprising detecting peaks of the Raman bands of thesubstance of interest when the laser source is exciting the sample atthe first wavelength, and detecting valleys of the Raman bands of thesubstance of interest when the laser source is exciting the sample atthe second wavelength.
 12. The method of claim 9, further comprisingproducing a background-free set of values representing true amplitudesof the Raman peaks of the substance of interest.